This invention relates generally to imaging lens systems and photo masks for optically defining patterns, and more particularly the invention relates to integrated circuit mask analysis and locating areas in a mask that are sensitive to residual processing effects. The invention has applicability in other image analysis, including signature analysis, for example.
In the fabrication of electronic integrated circuits, line patterns of less than a tenth of a micron are now being developed. This is less than 1/500 the width of a human hair. At these dimensions, the projection printing of mask patterns in integrated circuit layout can be adversely impacted by small residual aberrations in the lens system. While the quality metric (Strehl ratio) of today's projection printers is within a few percent of unity, residual aberrations still contribute significant spillover of signals from one mask opening to another. These spillover effects degrade the image quality with position within the field of the die. Good correlation of measured aberrations with the difference in horizontal and vertical linewidth along the slit in a scanning system has been observed. Such aberration-based linewidth variations are themselves partially mitigated by higher image slopes created through optical proximity correction (OPC). Yet residual cross-chip linewidth variations suggest that residual aberrations continue to contribute a level of degradation that is about half as large as the level of improvement gained through applying OPC. The impact of these aberration-based spillover effects will clearly become more important with phase shifting masks due to the inherent use of more coherent illumination as well as the presence of both phases to more efficiently direct energy to a broader set of locations in the lens pupil. Since lithography at low kl and high NA is adversely affected by small residual aberrations in lenses even though Strehl ratios exceed 0.98, test targets are needed to act as ‘canaries’ that are more sensitive than product features as well as to quantify individual aberrations to 0.01λ mms.
Co-pending application Ser. No. 10/241,242, supra, is concerned with characterizing a lens and lens system as to aberrations through the use of a probe and surrounding pattern by illuminating a mask having a small opening (probe) and a surrounding adjacent open geometry (pattern). The combined intensity pattern at the probe position near an image plane of the lens is then observed for spillover from the surrounding pattern that occurs as light passes through the lens. For this application, patterns corresponding to the inverse Fourier transform (IFT) of aberration representations used in characterizing lenses can be utilized. This gives an indication of the presence and level of lens aberrations, such as coma, astigmatism, spherical, and trifoil or even multiple terms in the Zernike representation. Measurements of the combined image compared to the individual image of the probe and pattern thus contain quantitative information on the level of aberrations. This intensity change can be observed directly by signal detection or indirectly by, for example in lithography, performing wafer exposures at various doses and comparing the dose at which the various parts of the image print in photoresist with the dose required to clear large areas or isolated probes.
The application also discloses a pattern-matching method for predicting worst case locations of residual aberration induced feature changes in the projection printing of large layouts including chip layouts used in integrated circuit manufacturing. A CAD system can rapidly determine locations in large layouts that are most impacted by aberrations in projection printing. For this application, aberrations are accurately modeled as producing spillover between mask openings with a localized pattern that is the inverse Fourier transform (IFT) of the optical path difference (OPD) function in the lens. The novel function in the CAD system then quickly rank orders all pattern edges and corners of interest according to the degree of similarity of their surrounding layout to the IFT function.
In carrying out the pattern matching the application discloses a bitmap matrix algorithm in which a bitmap of the layout is multiplied with the test pattern bitmap to compute the final match factor at each layout pixel of interest. The entire mask layout is represented as one huge bitmap of layers, similar to images on a computer screen. At every match location of interest (e.g., edge, corner) each pattern pixel is multiplied by the layout pixel at that coordinate and summed. However, the bitmap algorithm is too slow and too data intensive.
The present invention is directed to proving new algorithms based on edges, rectangles and triangles, which more efficiently compute match factors for a large number of test points.